1. Field of the Invention
The invention relate s to a coordinates input apparatus and, more particularly, to a coordinates input apparatus for applying a vibration to a vibration transmission plate, detecting a vibration to be transmitted by a plurality of vibration sensors arranged for the vibration transmission plate, and deriving input coordinates of the vibration on the basis of transmission times of the vibration.
2. Related Background Art
Hitherto, there has been known a coordinates input apparatus for inputting a vibration to a vibration transmission plate by a vibrator pen having therein a piezoelectric device or the like, detecting the vibration by a plurality of vibration sensors provided for the vibration transmission plate, and detecting coordinates of an input point.
In such a kind of coordinates input apparatus, a plate-shaped member of an elastic material such as glass, aluminum, or the like or a construction in which a resin sheet is laminated on the surface of a plate-shaped member as disclosed in JP-A-63-245711 is used as a vibration transmission plate.
In the above conventional apparatus, however, a sound wave having a large sound pressure among sound waves which are transmitted in the air gives a vibration to the vibration transmission plate, such a vibration is detected by the vibration sensor as noise together with the vibration inputted from the vibrator pen and it becomes a cause of occurrence of errors in a calculation result of the coordinates.
FIG. 15 is a cross sectional view showing a construction of a conventional coordinates input apparatus. As shown by an arrow of a bold line in the diagram, a vibration from a vibrator pen is transmitted in a vibration transmission plate and is detected by vibration sensors. There is, however, a case where a vibration generated from a sound source other than the vibrator pen is also transmitted in the air and enters the vibration transmission plate. The vibration which is transmitted in the air and is inputted is hereinafter also referred to as an xe2x80x9caerial transmitted vibrationxe2x80x9d.
The input of the vibration to the vibration transmission plate by the vibrator pen is executed by making a pen tip of the vibrator pen come into contact with the vibration transmission plate. In this case, since the vibration is transmitted from a solid to another solid, a matching performance of an acoustic impedance is good and an incident efficiency of the vibration is high. When the aerial transmitted vibration enters the vibration transmission plate, on the other hand, since the vibration is transmitted from gas to a solid, a matching performance of the acoustic impedance is bad and most of the vibration is reflected and an incident efficiency is low. Thus, an influence of noise with respect to the vibration of a low sound pressure is small.
As for the aerial transmitted vibration such as a voice of a person whose sound pressure is relatively low or the like, however, although an influence as noise is small, with respect to the aerial transmitted vibration of a mechanical sound, a burst sound, or the like in which a sound pressure is relatively high, an influence as noise is large and it becomes a cause of deterioration in calculating precision of coordinates.
In the conventional coordinates input apparatus, the vibrator pen is driven at a predetermined period and a transmission time of the vibration from the vibrator pen to each vibration sensor, namely, a vibration transmission time is measured. Therefore, when the aerial transmitted vibration of a high sound pressure is intermittent, so long as it differs from a timing when the vibration by the vibrator pen is detected at each vibration sensor, no influence is exerted on the detecting precision of the coordinates. However, when the vibration of a high sound pressure is continuously generated, an erroneous detection of the coordinates or a deterioration of the calculating precision occurs.
As a countermeasure for the aerial transmitted vibration, it is easily considered to provide a filter circuit for removing a frequency band of the aerial transmitted vibration which is detected by the vibration sensor. However, when the frequency band of the aerial transmitted vibration serving as a noise source is a wide band, a case where the frequency band of the vibration which is generated by the vibrator pen is included in the frequency band is presumed. A case where the frequency band of the aerial transmitted vibration is the same as the frequency band of the vibration that is generated by the vibrator pen is also presumed. Therefore, some of the filter circuits cannot remove the influence by the aerial transmitted vibration of the same frequency band as that of the vibration that is generated by the vibrator pen and cannot be regarded as effective means.
Although the construction such that the resin sheet or the like is adhered to the whole surface of the vibration transmission plate is effective with respect to the feeling of writing, a protection of the surface of the vibration transmission plate, a prevention of the reflection of the light, and a prevention of scattering when the vibration transmission plate is broken, it is not an effective countermeasure for the aerial transmitted vibration. This is because, for example, when the resin sheet is adhered to the surface of the vibration transmission plate, the resin sheet and the vibration transmission plate are acoustically adhered between solids by an adhesive layer, a matching performance of the acoustic impedance is good, and the aerial transmitted vibration which arrived at the resin sheet is easily propagated to the vibration transmission plate.
According to a preferred embodiment of the invention, it is an object to provide a coordinates input apparatus for applying a vibration to a vibration transmission plate by vibration input means, detecting the vibration to be transmitted by a plurality of vibration sensors arranged for the vibration transmission plate, and deriving input coordinates of the vibration on the basis of a transmission time of the vibration, comprising: a plate member having elasticity; and instructing means for instructing the plate member by providing a gap between the plate member and the vibration transmission plate.